Secondary hardening behavior in Ti alloy

We report the existence of secondary hardening behavior in Ti–10V–2Fe–3Al (wt.%) (Ti-1023) for the first time. Through controlling the ageing temperature window between 550 °C and 575 °C, alloys are found to show the existence of two hardness peaks with aging time. This heat treatment with secondary hardening phenomenon exhibits unusual increase of hardness and strength. Further experimental observations show that the first hardness peak corresponds with the well precipitated α phase at very short time. Further increase of aging time does not influence the morphology of these intragranular α precipitates, but accompanying with a continuous composition change, which could be the origin of the secondary hardness peak. This structure change followed by composition diffusion is the typical characteristic of pseudospinodal mechanism. Phase field and ab-initio calculations based on pseudospinodal mechanism analyze the detailed microstructure/composition evolution and related solid solution strength, which confirm the role of composition on the appearance of secondary hardening behavior.